Plasma Cutting - Dross Problems

[Zippo]

Plasma Cutting - Dross Problems

DROSS
Dross is re-solidified oxidized molten metal that is not fully ejected from the kerf during cutting. It is the most common cut quality problem of plasma cutting. Dross may form as a thick bubbly accumulation along the bottom edge of the plate (low speed dross), a small hard bead of uncut material (high speed dross) or a light coating along the top surface of the plate (top spatter).

FORMATION
Dross formation is dependent on many process variables including torch travel speed, standoff distance, amperage, voltage and consumable condition. It is also affected by material variables such as thickness and type of material, grade, chemical composition, surface condition, flatness, and even temperature changes in the material as it is cut. However, the three most critical variables to consider in dross formation are cutting speed, amperage, and standoff distance.


LOW SPEED DROSS
If the cutting speed is too slow, the plasma jet begins to look for more material to cut. The arc column grows in diameter, widening the kerf to a point where the high velocity portion of the plasma jet no longer ejects the molten material from the cut. As a result, this molten material begins to accumulate along the bottom edge of the plate in a thick globular form. This is called low-speed dross. At extremely low speeds the arc extinguishes because there is not enough metal to sustain a transferred arc. Increasing the amperage or decreasing the standoff (while keeping material thickness and speed constant) have a similar effect on the cut as slowing down the cut speed. Both of these changes cause more energy from the plasma jet to contact a given area of the material in a given period of time. Excessive amperage or low standoff can also cause low-speed dross. (Some low speed dross in the corners of a plasma cut is normal since velocity does not remain constant through a sharp turn.)


To eliminate low speed dross:

Increase the cut speed in 5 ipm increments

Increase the standoff in 1/16 increments or 5 volt increments

Decrease the amperage in 10 amp increments

If none of these measures improve the cut, consider a smaller nozzle size


HIGH SPEED DROSS
If the cutting speed is too fast, the arc begins to lag back in the kerf leaving a small hard bead of uncut material or rollover dross along the bottom of the plate. This high-speed dross is more tenacious and usually requires extensive machining to remove. At extremely high speeds the arc becomes unstable and begins oscillating up and down in the kerf causing a rooster tail of sparks and molten material. At these speeds the arc may fail to penetrate the metal or extinguish. High standoff or low amperage (for a given material thickness and cutting speed) can also cause high speed dross since both of these changes cause a reduction in the energy of the plasma jet.

To eliminate high speed dross:

Check the nozzle first for signs of wear (gouging, oversize or elliptical orifice)

Decrease the cutting speed in 5 ipm increments

Decrease the standoff in 1/16 increments or 5 volts increments

Increase the amperage (do not exceed 95% of the nozzle orifice rating)


TOP SPATTER DROSS
Top spatter is an accumulation of re-solidified metal that sprays along the top of the cut piece. It is usually very easy to remove. A worn nozzle, excessive cutting speed, or a high standoff is usually the cause. It is caused by the swirling flow of the plasma jet, which at a certain angle of attack flings molten material out in front of the kerf rather than down through it.

To eliminate top spatter dross:

Check the nozzle for signs of wear

Decrease the cutting speed in 5 ipm increments

Decrease the standoff in 1/16 increments or 5 volt increment.....

These are simple tips that will help your cut quality ... And of course every machine set up is just a little different than the next..
But even so the solutions are the same for dross....

[Largemouthlou]

Good info all in one spot!! Thanks.

[acourtjester]

thanks for posting filed away on disc

[billyeagley]

Thanks Zippo, that is good info!

[East German]

We all have the same problems with dross.

Thanks for the Information!

Regards Peter

[gamble]

At what speeds would you consider high vs low speed?
I cut a lot at 72IPM (aluminum, .100, 5052) so is that considered high or low?

[plain ol Bill]

72 IPM is slow really. What kind of cut parameters are you using to cut anyway?

[Largemouthlou]

I cut .090 at 133 ipm and .125 at 95 ipm with 40 amp HT fine cut.

Cut some paper thin aluminum for a guy the other day, first time ever so went with book specs of 395 ipm 25 amp.. cut pretty good, need to play with it a little more though.

[jimcolt]

Keep in mind that plasma torch and consumables designs vary considerably. Some torches will exhibit a very wide dross free speed range (DFI, or dross free interval), while others will have a non-existent DFI. I know that when Hypertherm process engineers design new plasma torches and consumable parts that months of testing......by cutting tons of steel sheet and plate....are done to develop designs with the widest DFI.

Also...different sheets/plates of metal will have varying chemistry....that directly affect the dross free speed interval. Carbon content, silicon content and manganese levels have a large effect of dross formation.

Oxygen plasma systems were designed a few decades ago in order to improve the ability to cut dross free. Industrial machines with oxygen based plasma are widely used for this reason. Do not attempt to use oxygen in an air plasma system....it is dangerous!

Jim Colt Hypertherm

[CNCCAJUN]

My 2 cent observation . . .

I have come to the conclusion, perhaps incorrectly that I can cut almost 100% dross free if I am able to cut at book specs.

There lies the problem . . . maintaining book specs . . .based on reading 100's of post about dross I am pretty sure no one can cut the "TREE OF LIFE" file in a 12" X 12" square of 16 gauge totally dross free.

Although that may happen the day that a plasma manufacturer realizes there is a market for that piece of equipment.

Look at the PowerMAX XP30 . . . It is getting pretty close . . .

I'm quiet sure that with competition and price drop of lasers they will start to see a drop off in sales of their high end plasma cutters. Hypertherm like several others are also in the laser business, so I am sure it would be detrimental to their laser future if you could buy a dross free air plasma unit that cut almost as well as a laser.

EXAMPLE:

Low Speed Fine Cut - Mild Steel

16 gauge - 40 amps - 0.06 torch height - 0.09 pierce height - 0.4 pierce delay - 150 IPM - 75 volts.
* This is not a dross free cut.

Problem - Now with the artsy stuff I have a 100 or more "CORNERS" to contend with.
My table like every one else's will drops below set speed on the "CORNERS".
Now I have left book specs and there is my cause of dross.

Fine Cut - Mild Steel

16 gauge - 45 amps - 0.06 torch height - 0.15 pierce height - 0.4 pierce delay - 250 IPM - 78 volts.
* No warning that this is not a dross free cut.

Problem - for one my max speed is 200 IPM.
Now with the artsy stuff I have a 100 or more "CORNERS" to contend with.
My table like every one else's will drops below this on the "CORNERS".
Now I have left book specs and there is my cause of dross.

As soon as I get away from the artsy stuff and start cutting parts with radiuses, lines & circles that are found on most parts I can cut roughly 95% dross free. What dross I do get I can usually brush off with my glove.

So I have decided to be happy with my "very accurate" clean part making, zero smoke in shop CNC plasma table.

When ever I cut artsy stuff I will use one of the dozen or so methods available to remove the dross.

Tired of chasing . ..

Steve

[davek0974]

I've pretty much given up trying to go dross free, I have the 30xp on my little 2x2 table and no matter what I do I always get dross, the only thing that changes is how much I get.

Most of my stuff is artsy so loads of swirls, loops, leaves and bends etc, the last one I did was 2.5mm steel and very artsy, the dross was that hard I ended up using an air chisel on some of it! Set on a very low pressure it's more like an air tap tap tap and is quite useful.

Generally on standard parts I can tap it off with a piece of scrap and it just falls of easy but either way I always get dross.

I tune for the fastest cut speed and use that as my spec for that material, I have a very good THC and run finecuts whenever possible, usually at 0.5 - 0.75mm cut height and 3mm pierce height.

As I said, I've pretty much given up trying to get dross free, seems a pointless battle.

[plain ol Bill]

You can cut almost dross free but getting all parts to the point you don't have to clean them is just a pita. Buy a brick chisel and a twist wire brush for your 4" grinder and enjoy life.

[jimcolt]

Lasers and high definition plasma cutters often produce dross......and in many cases it can be traced to torch height control, machine cut speed and or operator setup issues.

I will be surprised if air plasma systems ever cut 100% dross free......and in most cases that is due to the fact that they are used on "low cost" cnc machines (compared to the motion control platform used on Laser and High Definition plasma). Machines that have lesser acceleration, less accurate linear ways, and less accurate means of controlling the torch to work distance.....will always have more variability in cut quality consistency. Ad that to the simple fact that an air plasma system is designed to be affordable (a Hypertherm air plasma rated to pierce and cut 3/4" steel is about 1/10th the purchase price of a high definition plasma with the same thickness capacity). Air plasma uses a gas cooled torch (vs liquid) uses the "universal plasma gas (air).....and must be portable and low cost (customer demand).

On my home machine (Powermax85 , PlasmaCam cnc machine) I get the cut specs dialed in to minimize dross to the point that I do not need to touch the part with a grinder.....however in the majority of the cuts I need to do a bit of dross removal in the form of scraping or wire brushing. If the part is relatively simple....without a lot of intricate detail that causes the machine to slow down (building heat and dross)....then I can get a 99% (or so) dross free part.

Jim Colt

[CNCCAJUN]

On my home machine (Powermax85 , PlasmaCam cnc machine) I get the cut specs dialed in to minimize dross to the point that I do not need to touch the part with a grinder.....however in the majority of the cuts I need to do a bit of dross removal in the form of scraping or wire brushing. If the part is relatively simple....without a lot of intricate detail that causes the machine to slow down (building heat and dross)....then I can get a 99% (or so) dross free part.
Jim Colt

It is the same for me . . . cuts pieces parts - no dross to speak of . . . . cut intricate artsy stuff - grab grinder .

It has become very clear to me, "IF" I had a cnc plasma table capable of moving @ 350 IPM with total accuracy, requiring zero slow down in corners or direction change while maintaining a perfect THC I would have NONE or VERY LITTLE DROSS regardless of what gauge or file I was cutting. I realize the Hypertherm is capable of doing this, but it needs the above platform to support it.

So I can spend another $50.000.00 + dollars to accomplish the above or I can use one of the dross removal methods.

While conceding the above, I still think we will see in the near future an air plasma system that will be capable of getting us closer to the goal of reduced dross for the 1000's of <$20,000.00 tables out there.

All things considered, $$$ being one, I am still very pleased with my Dynatorch as well as my PowerMAX 85. . .

Steve